In recent years, as miniaturized, high-speed, high-frequency electronic apparatuses have been introduced, circuit components used in such apparatuses need to have characteristics usable in a range from several hundreds of MHz to several GHz. For example, impedance elements for noise reduction need to maintain high impedance in the above high frequency band. As another example, communication apparatuses, such as mobile phones and wireless LANs, operate in a frequency band ranging from several hundreds of MHz to several GHz and need to be able to operate in a wide band and to have high efficiency in that band. Antennas used in such communication apparatuses also need to function with high gain in the above band in the first place, and need to be compact and low-profile from the viewpoint of their usage. Furthermore, in the digital terrestrial broadcasting, which has recently commenced, antennas to be used need to cover a wide frequency band ranging from 470 MHz to 770 MHz in order to receive all channels. In the present application, it is noted that the description of “a to b” means “equal to or greater than a and equal to or smaller than b.”
As a compact, low-profile antenna suitable for the above-mentioned communication apparatuses, such as mobile phones, a so-called chip antenna based on dielectric material has been conventionally used (Patent Reference 1). At a fixed frequency, use of dielectric material with a higher dielectric constant allows reduction in the size of the chip antenna. There has also been proposed a chip antenna using magnetic material with high magnetic permeability as well as a high dielectric constant to reduce the size of the antenna (Patent Reference 2).
When magnetic material is used for a chip antenna, for example, spinel ferrite, such as Ni—Zn ferrite, has so-called Snoek's limit, which limits use of spinel ferrite in high frequency regions. In contrast, since hexagonal ferrite has an easy magnetization axis in a plane perpendicular to the c axis and hence maintains predetermined magnetic permeability even in a frequency b or higher than the frequency limit of spinel ferrite (Snoek's limit), so that hexagonal ferrite has been proposed as one of magnetic materials for chip antennas (Patent Reference 3). Patent Reference 3 discloses that Z-type ferrite in which most of particles have a single domain structure is particularly used in a high frequency region in an effective manner.
Although not particularly limited to the chip antenna application, Patent Reference 4 discloses, as a material for high frequency circuit components, magnetic oxide sintered body mainly made of Y-type hexagonal ferrite, occupying at least 80% of the composition, for the purpose of increasing the resistivity and lowering the dielectric constant. The magnetic oxide sintered body contains cobalt oxide, copper oxide and iron oxide as main components and AO (AO stands for at least one of BaO and SrO) for the remaining part, and also contains borosilicate glass and the like as sub-components.
Patent Reference 1: JP-A-2001-358516
Patent Reference 2: JP-A-49-40046
Patent Reference 3: WO 96/15078
Patent Reference 4: JP-A-2002-260912